User input interfaces are used in many different types of electronic devices and generally allow human users to enter data and/or commands into an electronic format. A keypad is a type of user input interface in which one or more keys or buttons are pressed to enter the data and/or commands. In a typical keypad, the keys are movably mounted such that application of a force to a key causes the key to move relative to another portion of the keypad. Movement of the key can then be sensed electrically, for example, by causing electrical continuity to occur within a circuit. The force that moves the keys on a keypad is typically provided by the human user's finger.
Keypads are used in many other applications, in addition to cellular telephone devices. Examples of such other applications that typically utilize keypads include electronic dictionaries, personal digital assistants (PDAs), handheld game consoles, security input pads for allowing selective entry into a facility or area and remote control devices for video and audio appliances.
A keypad may include multiple keys, each corresponding to a different command and/or data input selection. An example of a keypad with multiple keys is the type of keypad commonly used in cellular telephone devices. In a cellular telephone device, the keypad may, for example, include individual keys for each of the digits 0 through 9 to allow telephone numbers to be entered into the cellular telephone device. To help a human user interface with an electronic device via a keypad, the keys on a keypad typically include icons to identify what command or data input selection each key represents. In the example of a cellular telephone device keypad, as discussed above, the keys for each of the digits 0 through 9 may each include an appropriate icon indicating the digit that each key represents (the key that corresponds to 0 would include the icon “0”, and so on). Keys often may serve different functions or data input selections when, for example, the device is in different modes of operation.
In order to facilitate use of a keypad in low ambient light situations, many keypads include a backlighting feature in which one or more light sources illuminates the keys from beneath. This backlighting generally causes each key to be lighted so that it can readily be found and identified. In many cases, the keypad icons are provided as relatively transparent features so that the icons, themselves, will be illuminated, thus further facilitating use of the keypad in low ambient light situations.
Electronic devices with backlit keypads typically include, for example, a keypad assembly having a keypad, a switchboard below the keypad and a light guide between the keypad and the switchboard. A number of light emitting diodes (LEDs) are normally located adjacent the light guide to supply light into the light guide. The keypad is commonly constructed as an elastic pad plate with a plurality of key buttons formed therein. As discussed previously, each of the key buttons may include one or more icons or characters printed on the upper surface thereof. Each of the key buttons generally also includes one or more protrusions (or actuators) formed on its lower surface.
In many cases, the switchboard takes the form of a printed circuit board with a plurality of switches formed on the upper surface thereof facing the keypad. Each of the switches may, for example, include a conductive contact member and a conductive dome covering the contact member. When a user presses one of the key buttons, the portion of the keypad positioned beneath the pressed key button deforms towards the switch board. As a result, one of the protrusions corresponding to the deformed portion presses the corresponding conductive dome on the switchboard, thus providing an electrical contact with the corresponding contact member.
To enhance optimum usability of keypad devices in low ambient light situations, it is desirable to provide keyboard backlighting that is both sufficient in intensity and relatively uniform in distribution across the illuminated keys.